Phase shift key (PSK) modulation of radio signals has been used in the past to transmit digital information between data processing systems. One example is shown in U.S. Pat. No. 5,150,070, entitled "Demodulator for biphase, suppressed-carrier PSK signals" by P. Rinaldi. The phase modulation technique uses a 180 degree phase shift to distinguish between a binary one and a binary zero. This forces the carrier to be zeroed-out during modulation. To demodulate the modulated signal, the prior art requires complex circuitry to reliably reconstitute the binary information at the receiver. The demodulators of the prior art must reconstruct the carrier. They require coherent demodulation to create a signal that is phase locked with the incoming signal, and they then must combine the two in a multiplier to detect the data. Consequently, the IF signal must be made synchronous with the demodulated signal off the carrier. Stated otherwise, the carrier and the local oscillator must be made synchronous to demodulate the PSK signal.
In phase shift key modulation, a carrier signal, for example a 2.4 GHz carrier signal, is selectively applied to a phase shift delay circuit, depending upon the binary state of control input to the delay circuit. For example, when there is a binary zero data state for the control input, no phase shift delay is applied to the carrier signal. Alternately, when there is a binary one data state, a phase shift delay is applied to the carrier signal. The carrier signal is then transmitted to the receiver. At the receiver, there is a local oscillator that oscillates at a slightly different frequency, for example 2.4 GHz plus 2 MHz. At the receiver, these two frequencies are mixed and a corresponding beat note signal, or intermediate frequency (IF) signal, is produced. Phase shift information, which has been imposed on the carrier signal, is then manifested in the IF signal at the receiver. A significant problem in such phase shift key modulation communication techniques is created by the drift in the frequency of the oscillator at the transmitter which generates the 2.4 GHz carrier signal, and the drift of the local oscillator at the receiver which generates the 2.4 GHz plus 2 MHz signal. The relative drift in the frequencies of these two oscillators results in unstable characteristics in the intermediate frequency produced at the receiver and, therefore, unreliable detection of the binary data being transmitted.
U.S. Pat. No. 5,561,689, filed Oct. 26, 1994, by Fleek et al., discloses a phase demodulation technique for wireless LAN that allows the carrier and the local oscillator to not be synchronous. A PSK modulated signal received at a receiving station is amplified by a limit amplifier to form a square wave signal having pulses of uniform height. To detect a carrier sense condition, the time between the rising edges of the square wave IF signal that occur in response to the modulation at the transmitter are detected using a counter during a carrier sense measurement window. Times between the like edges of the square wave signal that are shorter than normal for an unmodulated IF signal indicate a first binary value of the modulation. Longer than normal times between the like edges of the square wave IF signal indicate a second binary value.
To compensate for drift in the carrier and in the local oscillator frequencies, the nominal non-phase transition portions of the IF signal are periodically sampled and corresponding registers are updated. Nevertheless, for a reliable carrier sense indication the IF frequency of the receiving station is required to be constant for a large number of cycles. This requirement greatly constrains the difference in crystal references between the transmitting and receiving stations and makes the measurement window for detecting a carrier sense indication relatively large. Other factors associated with this particular approach that contribute to the requirement of a large measurement window are, for example, the phase noise of the voltage controlled oscillator (VCO) of the local oscillator (LO), and variations in the LO frequency that are caused by the phase locked loop (PLL) not being completely settled. By making carrier sense measurement window sufficiently large to allow for these factors, this particular technique occasionally provides false carrier sense indications, adversely affecting the reliability of the collision avoidance algorithm provided by the Media Access Control layer of the wireless LAN system. For example, after a crier sense condition is active, the measurement window is expanded to allow for the variations in the IF that are caused by the Phase Shift Modulation. As a result, a carrier sense indication occasionally remains active after transmission of a data frame is complete.
What is needed is a demodulation technique that permits the crystal references at both the transmitting and receiving stations of a wireless network to have a greater frequency tolerance, %hat prevents slow changes in the LO caused by PLL settling time to adversely affect carrier sense indications, and that reliably senses the end of a data frame and loss of the carrier.